This invention is directed to transparent coatings for substrates and particularly to transparent coatings that are physically and chemically resistant.
Glass and similar transparent substrates can be coated with transparent films to alter the optical properties of the glass. High visible transmittance, low emissivity coatings are characterized by their ability to transmit visible light while minimizing the transmittance of other wavelengths of light, particularly light in the infrared spectrum. This characteristic is useful for minimizing radiative heat transfer without impairing visibility, and coatings of this type find utility in architectural glass or automobile windows. It is often desired to maintain reflectance relatively consistent throughout the visible spectrum so that the coating has a xe2x80x9cneutralxe2x80x9d color; that is, colorless.
Generally speaking, coatings on glass that are provided for high transmittance and low emissivity comprise a stack of films having one or more thin metallic films with high infrared reflectance and low transmissivity that are disposed between antireflective dielectric layers that commonly are metal oxide films. The metal oxide layers serve to reduce visible reflection of the film stack to enhance transmittance, and are characterized by relatively high indices of refraction, commonly on the order of 1.9 or more.
Thin, transparent metal films of silver, copper and the like are susceptible to corrosion (e.g., staining) when they are brought into contact, under moist or wet conditions, with various staining agents such as atmosphere-carried chlorides, sulfides, sulfur dioxide and the like. Films of this type commonly are employed on inner surfaces of multi-pane glass units so that the films are maintained in a dry condition by desiccants or the like that remove moisture from the interpane spaces. Staining can occur when coated panes of glass are stored for later fabrication into insulating glass units.
Film stacks frequently are isolated from contact with the environment, and a film stack of the type described may be positioned on one of the inner surfaces of a multipane insulating glass unit. However, when glass panes bearing coating stacks are transported or assembled into multipane units, they often are subjected to relatively harsh conditions which may cause physical marring of the film stacks.
Film stacks commonly are provided on glass sheets on a commercial production basis through the use of magnetron sputtering techniques such as those described in Chapin, U.S. Pat. No. 4,166,018.
Gillery, et al. U.S. Pat. No. 4,786,563 suggests the use of a thin overcoat of titanium dioxide as a protective layer. Titanium oxide overcoats may be particularly prone to scratching or abrasion during shipping and washing operations, however, rendering the glass panes commercially unsuitable for use. O""Shaughnessy, et al. U.S. Pat. No. 5,296,302 corrects the problem by providing a protective overcoat of an oxide such as zinc oxide, the latter being relatively very thin in comparison to other films in the stack, and protective overcoats of this type having thicknesses in the range of 10-40 xc3x85 are disclosed.
High transmittance, low emissivity film stacks of the type described in U.S. Pat. No. 5,296,302, despite their excellent resistance to scratching, nonetheless have experienced problems in connection with the tarnishing or other discoloration of the reflective metal layers, which commonly are silver. Moreover, since the sputter deposition of certain films such as titanium oxide proceeds more slowly than zinc oxide, for example, it would be desirable to avoid the presence of titanium oxide films of thicknesses greater than, for example, 30 xc3x85.
It would be desirable to provide such film stacks with protection not only against physical damage (e.g., scratching) but also against tarnishing or discoloration of the metal reflective layers employed in such film stacks.
We have found that application of a thin sputtered-on film of a compound of silicon and nitrogen such as silicon nitride as a protective film in a film stack provides the underlying metal (e.g., silver) metal film(s) with excellent resistance to corrosion while at the same time providing the underlying stack with resistance to physical marring, all without substantial effect upon optical properties of the stack. Preferably, the film directly beneath the silicon nitride protective film is a dielectric film having an index of refraction substantially the same as that of silicon nitride, that is, about 2.0xc2x10.1. The film directly beneath the silicon nitride film may be of a metal oxide such as an oxide of zinc or tin or alloys thereof, these oxides having substantially the same index of refraction. In this manner, the optical properties of a given film stack may be adjusted by adjusting the thickness of the contiguous, combined zinc oxide/silicon nitride films. Desirably, the silicon nitride film is at least 10 xc3x85 thick, and is present at a thickness less than 150 xc3x85, preferably not greater than 100 xc3x85 and most preferably not greater than about 50 xc3x85. Although the film stack may have thin (usually not greater than about 20 xc3x85) sacrificial or shielding films of titanium compounds such as titanium oxide, the stack desirably is free of titanium-containing films having thicknesses greater than about 30 xc3x85.
In one embodiment, the invention comprises a transparent support such as glass having a transparent film stack sputter deposited upon it, the film stack comprising a reflective metal film, a dielectric film over the metal film, and a thin (10 xc3x85-150 xc3x85) protective silicon nitride film over the dielectric film. In another embodiment, the invention relates to a transparent article having a pair of reflective metal films separated by a dielectric film, the film stack including an outer dielectric film and a thin protective film of silicon nitride over the outer dielectric film. Desirably, the dielectric film immediately beneath the protective silicon nitride film is a metal oxide film such as zinc oxide or tin oxide that has an index of refraction substantially the same as that of the silicon nitride so that the outer dielectric film and the silicon nitride film can be treated as a single film for adjusting optical properties of the stack.